The invention concerns a method for echo cancelling in a communication line system, particularly an integrated services digital network, abbreviated with ISDN, or any digital subscriber line , abbreviated with XDSL, system. Echo cancelling is normally performed in the analog front end of the communication line system, more particularly in between the line transformer and the analog/digital and digital/analog converters. The analog front end is part of the interface between the two-wire line and the digital transmitting and receiving devices coupled to it. It is known to realise echo cancelling by means of digital filter techniques.
Another known method consists in the synthesis of a digital hybrid impedance at the digital side of the A/D converter.
U.S. Pat. No. 5,287,406 discloses such method. A digital balancing circuit for cancelling a return echo is operatively connected to the two-wire/four-wire conversion means.
These digital methods for echo cancelling can however not sufficiently remove inherent non-linear distortion originating within the analog front end itself in the RX and TX paths.